1. Field of the Invention
The invention under consideration relates to a heat-generating element of a heating device for heating air, comprising at least one PTC element and, lying on opposing side surfaces of the PTC element, electric strip conductors. Such a heat-generating element is known, for example, from EP 1 061 776, which is traced back to the current applicant.
In particular, the heat-generating element is deployed in an auxiliary heater for a motor vehicle, and comprises multiple PTC elements, arranged in a row, one behind the other, that are energized via electric strip conductors that extend parallel to one another and that lie flat on opposing sides of the PTC elements. The strip conductors are normally formed by parallel strips of metal. The heat-generating elements formed in this way are deployed in a heating device for heating air in a motor vehicle, where said heating device comprises multiple layers of heat-generating elements having heat-emitting elements that lie on their opposing sides. These heat-emitting elements are positioned so that they lie against the heat-generating elements in a relatively good heat-transferring contact by means of a holding device.
2. Description of the Related Art
In the case of the aforementioned state of the art, a holding device of the heating device is formed by a frame in which multiple layers of heat-generating and heat-emitting elements that run parallel to one another are held by means of a spring bias. In an alternative development, which likewise discloses a generic heat-generating element and a generic heating device and that is described, for example, in EP 1 467 599, the heat-generating element is formed by multiple PTC elements arranged one behind the other, in a row in one level, said PTC elements also being called ceramic elements or positive temperature coefficient thermistors, and being energized on opposing side surfaces by strip conductors that lie on these side surfaces. One of the strip conductors is formed by a circumferentially closed profile, and the other strip conductor by a strip of metal that is supported at the circumferentially closed metal profile with an electrically insulating layer in between. The heat-emitting elements are formed by segments arranged in multiple parallel layers, said segments extending at right-angles to the circumferentially closed metal profile. In the generic heating device known from EP 1 467 599, multiple circumferentially closed metal profiles formed in the manner described in the preceding are provided, said metal profiles being arranged parallel to one another. To some extent, the segments extend between the circumferentially closed profiles and project beyond them to some extent.
In the case of the aforementioned heat-generating elements, there is a requirement that the electric strip conductors must be in good electrical contact with the PTC elements. Otherwise, the problem that arises is an increased transition resistance, which, particularly in the case of the use of heat-generating elements in auxiliary heaters for motor vehicles, can lead to local overheating due to the high currents. As a result of this thermal event, the heat-generating element can be damaged. Furthermore, the PTC elements are self-regulating resistance heaters that emit a lower heat output at an increased temperature, so that local overheating can lead to a disturbance in the self-regulating characteristics of the PTC elements.
In addition, at the high temperatures in the area of an auxiliary heater, vapours or gases can develop that can result in a direct hazard for persons in the passenger compartment.
Correspondingly problematic is also the use of generic heat-generating elements at high operating voltages, such as voltages up to 500 V, for example. For one thing, a problem here is that the air that flows against the heat-emitting elements carries moisture and/or dirt with it, which can penetrate into the heating device and cause an electric flashover, i.e., a short-circuit, here. On the other hand, there is fundamentally the problem of protecting persons working in the area of the heating device from the current-carrying parts of the heating device or of the heat-generating element.
In the case of heat-generating elements of the generic type, the PTC elements are usually arranged in a positioning frame that extends as a flat component essentially in the level of the PTC elements. The positioning frame serves the accurate positioning of the PTC elements during the assembly of the heat-generating element, and optionally also for holding the PTC elements during long-term operation. Because the positioning frame is made of plastic as an injection-moulded part, it consequently has certain insulating characteristics. It has been seen, however, that in generic heat-generating elements when high voltages are used, an electric flashover cannot always be avoided, due to a low resistance to leakage current.
In the state of the art, there has not been a lack of proposals for screening the PTC heating elements against the surroundings. For example, DE 32 08 802 discloses a heat-generating element with a positioning frame and PTC heating elements arranged therein, with said heating elements being sandwiched between opposing strip conductors and this heat-generating element being surrounded by a metallic capsule that is provided with an insulating silicone rubber hose on its interior side, so that the metallic capsule is not in direct electrical contact with the strip conductors. This heat-generating element serves the use in household appliances, press plates and the like, and is incorporated into a press plate for uniform dissipation of the heat generated in the heating element. In the case of this state of the art, the problem that exists is that uniform contacting between the strip conductors and the PTC elements cannot always be guaranteed. In addition, protection of the PTC elements against air and moisture, i.e., the flashover protection, is effected solely by the capsule that completely encloses the PTC elements, which complicates the manufacture of the heat-generating elements and which cannot be used for all conceivable applications of the heat-generating elements, particularly in the case of the use of heat-generating elements in an auxiliary air heater in a motor vehicle.
A heat-generating element is known from U.S. Pat. No. 4,327,282 that is realized without positioning frame and with which the PTC elements, which are arranged behind one another in each case, together with the conducting plates that lie on these elements on both sides and that form the strip conductors and the insulating layers arranged on their exterior sides are held on the long sides. By means of this holding of the layer composition on the long sides, adequate contacting should be effected between the PTC elements and the strip conductors. The mechanism for holding the layer composition on the sides is formed by U-shaped silicone profiles, whose flanges should lie on the insulating layer. It has been seen, however, that in this way, it is not possible to achieve adequate protection of the PTC elements against penetrating moisture and air, particularly when the heat-generating elements are used in an auxiliary air heater in a motor vehicle. The silicone strips are furthermore relatively soft and can be detached easily, for example, during assembly or repair work on the auxiliary heater. In an alternative solution proposal, known from EP 0 026 457, the PTC heating element is located within a layer composition, whose outer layers are each formed by an aluminium oxide layer, which outer layers clamping a strip conductor between themselves and the PTC heating element. The aluminium oxide plates are supported along the edges on a rigid plastic frame. The strip conductor is formed by a layer of ductile solder. The application of such a solder layer leads to manufacturing difficulties, however. Furthermore, during operation of the heat-generating element, the problem arises that the solder liquefies in an impermissible manner and produces a short-circuit within the heat-generating element. Due to the rigid support of the aluminium oxide plates on the plastic frame, the known heat-generating element furthermore lacks the ability of resiliently reacting to thermal expansions within certain limits, so that in the case of this state of the art, it is not possible to guarantee secure contacting between the strip conductors and the PTC heating element at all times. The corresponding applies to the heat-generating element known from US 2003/0206730, in which exterior aluminium oxide plates likewise lie on a frame that surrounds the PTC elements.
In the case of the heat-generating element known from U.S. Pat. No. 6,178,192, the PTC element, which is sandwiched between two strip conductors, is completely surrounded by an insulating casing that is formed from an electrically non-conductive plastic, so that, due to the poor thermal conductivity of the plastic material, heat dissipation away from the PTC heating element is hindered. Furthermore, limits are set for the effort to form the casing with a very low wall thickness, because otherwise the problem that occurs is that the casing becomes penetrable, as a result of which the circumferential insulation around the PTC element is destroyed. The moulding of the layer composition of strip conductors and PTC elements also represents a time-consuming manufacturing step, which additionally requires hardening or cooling times, as a result of which the manufacturing is additionally slowed down.